The fabrication of many types of paper, board, fabric, and other roll materials usually includes a drying stage for drying a web of material formed in a wet process. Many different systems have been devised for such drying of roll materials. Commercial systems typically have employed one or more heated, rotating drums and a porous wire or felt belt for supporting a web fed between the heated drum surface and the belt. However, web breaks have been a problem with such systems, particularly with lighter webs. If the drums were heated to high temperatures to effect a higher drying rate, uneven drying and curling of the roll material could result. The older systems also do not provide desired high strength properties of the web.
Other systems, such as shown in U.S. Pat. No. 3,503,139 of Mahoney, have employed dual rows of alternating high temperature (heating) and low temperature (cooling) drums, with an entrained wire or felt belt for transport of the web in a serpentine path around the drums, in order to effect a more gradual evaporation of water from the web. However, these systems had problems with breaking of the web as it is repeatedly moved in and out of contact with the high and low temperature drum surfaces. Some improved systems, such as shown in U.S. Pat. No. 3,925,906 of Chance et al., have employed two wire or felt belts in parallel for holding the web more stably between them as it is transported around the drums. Heated belts combined with porous drums for removal of vapor have also been used. However, the maintenance of an adequate transfer of heat and evenness of drying of the web along the transport path around the drums has been a major problem. Another approach has been to transport the web held between belts along a linear path between stationary heated elements, but this raises other problems in terms of adequate ventilation and vapor removal from the web path.
One recent system, shown in U.S. Pat. No. 4,622,758 of Lehtinen et al., has employed transport of the web between a heated metal belt and a felt belt around a cooling cylinder for evaporating water from the web for condensation into the felt belt and onto the cooling cylinder. As illustrated in FIG. 4 of the drawings, this system of Lehtinen et al. has a metal belt 6 entrained around four spaced-apart heated drums 1, 3, 4, 5. A paper web 8 is transported by a wire or felt belt 9 to a nip with the metal belt 6 around the first drum 1. The web held between the belts is then transported around the cooling cylinder 2 where a semi-circular air pressure chamber 16 is used to apply pressure to increase the temperature of the evaporated steam and to force evaporation of water from the web. Condensate is removed by a scraper 13 from the drum, while condensate in the wire or felt belt 9 is removed by a downstream suction box 23. After leaving the cooling cylinder, the web 8 is separated from the belts and passed to a second drying unit.
The system of Lehtinen et al. suffers from the following drawbacks. The semi-circular air pressure chamber 16 can exert excessive loading on the rotating cooling cylinder 2, and the system requires a second pressure box 20 located opposite the first one to counterbalance the load on the cylinder. As the web leaves the area of the pressure chamber 16 and the cooling cylinder 2, the external pressure on the web is suddenly released, while the temperature and vapor pressure within the web remains high. As a result, the vapor pressure can explode outward, thereby causing damage or at least delamination to the web. In addition, since the high pressure of the pressure chamber 16 and the high temperature of the heated metal belt 6 are applied only to one side of the web 8, and low temperature exists at the other side, the material on the heated side dries first, leading to non-uniform densification and resistance to further drying on that one side of the web. When the web is removed from the belt, the steam has to go through the densified layer, thereby leading to delamination. Further, removal of the web 8 from the metal belt 6 after leaving the cooling cylinder 2 can lead to sticking and breaks in the web as it is separated and transported to subsequent stages.
The temperature of the cooling cylinder has to be kept low in order to obtain the necessary condensation of moisture. The intensity of drying has heretofore been controlled by the temperature of the cylinders, but because of the very high, heat inertia of the cylinders, control by cylinder temperature has proven to be slow and inefficient. The condensed vapor will also become collected in the voids of the wire belt, and will limit the amount of water that can be removed from the web. This may require increased thickness of the wire, and may cause secondary re-wetting of the web after it leaves the cylinder. Therefore, the problems of non-uniformity of drying and uneven web densification have been difficult to solve.